Summary

Testing for business logic flaws in a multi-functional dynamic web application requires thinking in unconventional methods. If an application's authentication mechanism is developed with the intention of performing steps 1, 2, 3 in that specific order to authenticate a user. What happens if the user goes from step 1 straight to step 3? In this simplistic example, does the application provide access by failing open; deny access, or just error out with a 500 message?

There are many examples that can be made, but the one constant lesson is "think outside of conventional wisdom". This type of vulnerability cannot be detected by a vulnerability scanner and relies upon the skills and creativity of the penetration tester. In addition, this type of vulnerability is usually one of the hardest to detect, and usually application specific but, at the same time, usually one of the most detrimental to the application, if exploited.

The classification of business logic flaws has been under-studied; although exploitation of business flaws frequently happens in real-world systems, and many applied vulnerability researchers investigate them. The greatest focus is in web applications. There is debate within the community about whether these problems represent particularly new concepts, or if they are variations of well-known principles.

Testing of business logic flaws is similar to the test types used by functional testers that focus on logical or finite state testing. These types of tests require that security professionals think a bit differently, develop abused and misuse cases and use many of the testing techniques embraced by functional testers. Automation of business logic abuse cases is not possible and remains a manual art relying on the skills of the tester and their knowledge of the complete business process and its rules.

Business Limits and Restrictions

Consider the rules for the business function being provided by the application. Are there any limits or restrictions on people's behavior? Then consider whether the application enforces those rules. It's generally pretty easy to identify the test and analysis cases to verify the application if you're familiar with the business. If you are a third-party tester, then you're going to have to use your common sense and ask the business if different operations should be allowed by the application.

Sometimes, in very complex applications, the tester will not have a full understanding of every aspect of the application initially. In these situations, it is best to have the client walk the tester through the application, so that they may gain a better understanding of the limits and intended functionality of the application, before the actual test begins. Additionally, having a direct line to the developers (if possible) during testing will help out greatly, if any questions arise regarding the application's functionality.

Description of the Issue

Automated tools find it hard to understand context, hence it's up to a person to perform these kinds of tests. The following two examples will illustrate how understanding the functionality of the application, the developer's intentions, and some creative "out-of-the-box" thinking can break the application's logic. The first example starts with a simplistic parameter manipulation, whereas the second is a real world example of a multi-step process leading to completely subvert the application.

Example 1:

Suppose an e-commerce site allows users to select items to purchase, view a summary page and then tender the sale. What if an attacker was able to go back to the summary page, maintaining their same valid session and inject a lower cost for an item and complete the transaction, and then check out?

Example 2:

Holding/locking resources and keeping others from purchases these items online may result in attackers purchasing items at a lower price. The countermeasure to this problem is to implement timeouts and mechanisms to ensure that only the correct price can be charged.

Example 3:

What if a user was able to start a transaction linked to their club/loyalty account and then after points have been added to their account cancel out of the transaction? Will the points/credits still be applied to their account?

Business Logic Test Cases

Every application has a different business process, application specific logic and can be manipulated in an infinite number of combinations. This section provides some common examples of business logic issues but in no way a complete list of all issues.

In business logic data validation testing, we verify that the application does not allow users to insert “unvalidated” data into the system/application. This is important because without this safeguard attackers may be able to insert “unvalidated” data/information into the application/system at “handoff points” where the application/system believes that the data/information is “good” and has been valid since the “entry points” performed data validation as part of the business logic workflow.

In forged and predictive parameter request testing, we verify that the application does not allow users to submit or alter data to any component of the system that they should not have access to, are accessing at that particular time or in that particular manner. This is important because without this safeguard attackers may be able to “fool/trick” the application into letting them into sections of thwe application of system that they should not be allowed in at that particular time, thus circumventing the applications business logic workflow.

In integrity check and tamper evidence testing, we verify that the application does not allow users to destroy the integrity of any part of the system or its data. This is important because without these safe guards attackers may break the business logic workflow and change of compromise the application/system data or cover up actions by altering information including log files.

In process timing testing, we verify that the application does not allow users to manipulate a system or guess its behavior based on input or output timing. This is important because without this safeguard in place attackers may be able to monitor processing time and determine outputs based on timing, or circumvent the application’s business logic by not completing transactions or actions in a timely manner.

In function limit testing, we verify that the application does not allow users to exercise portions of the application or its functions more times than required by the business logic workflow. This is important because without this safeguard in place attackers may be able to use a function or portion of the application more times than permissible per the business logic to gain additional benefits.

In circumventing workflow and bypassing correct sequence testing, we verify that the application does not allow users to perform actions outside of the “approved/required” business process flow. This is important because without this safeguard in place attackers may be able to bypass or circumvent workflows and “checks” allowing them to prematurely enter or skip “required” sections of the application potentially allowing the action/transaction to be completed without successfully completing the entire business process, leaving the system with incomplete backend tracking information.

In unexpected file upload testing, we verify that the application does not allow users to upload file types that the system is not expecting or wanted per the business logic requirements. This is important because without these safeguards in place attackers may be able to submit unexpected files such as .exe or .php that could be saved to the system and then executed against the application or system.

In malicious file upload testing, we verify that the application does not allow users to upload files to the system that are malicious or potentially malicious to the system security. This is important because without these safeguards in place attackers may be able to upload files to the system that may spread viruses, malware or even exploits such as shellcode when executed.

Tools

While there are tools for testing and verifying that business processes are functioning correctly in valid situations these tools are incapable of detecting logical vulnerabilities. For example, tools have no means of detecting if a user is able to circumvent the business process flow through editing parameters, predicting resource names or escalating privileges to access restricted resources nor do they have any mechanism to help the human testers to suspect this state of affairs.

The following are some common tool types that can be useful in identifying business logic issues.

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